Wireless sensor networks (WSNs) are a prominent fundamental technology of the Internet of Things (IoTs). Rather than device-to-device communications, group communications in the form of broadcasting and multicasting incur efficient message deliveries among resource-constrained sensor nodes in the IoT-enabled WSNs. Secure and efficient key management is in many cases used to protect the authenticity, integrity, and confidentiality of multicast messages. This paper develops two group key establishment protocols for secure multicast communications among the resource-constrained devices in IoT. Major deployment conditions and requirements of each protocol are described in terms of the specific IoT application scenarios. Furthermore, the applicability of the two protocols is analyzed and justified by a comprehensive analysis of the performance, scalability, and security of the protocols proposed.